Although low sulfur, sub-bituminous coal is readily available it may have such a high moisture content and low heating value that it is of little use as a fuel in existing boilers. Thermal drying to reduce the moisture content of the coal can upgrade its heating value to a point where the dried coal can compete favorably with many bituminous coals. With a low sulfur content such coal can meet clean air requirements for many power plants and make a major contribution to reducing sulfur dioxide emissions and acid rain. The drying required with such low rank coals is a deep drying process to remove both surface water and large quantities of interstitial water present. The handling, storage and transportation of such deep dried coal can present technical problems resulting from the friability and dustiness of the coal, as well as its tendency to readsorb moisture and react with oxygen from the air. Spontaneous combustion can result from heats of moisture readsorption and oxidation. Removing moisture inherent in the coal structure can also reduce the strength of the coal particle by cracking or fissuring, causing friability and dusting. The number of active surface sites exposed within the coal particles can also thus be increased, thereby increasing undesired moisture adsorption and oxidation.
Drying conditions such as temperature, residence time within the drying chamber, drying gas velocities, etc., affect the tendencies of the dried coal product to exhibit the undesirable qualities discussed above. For example, rapid removal of moisture by a high drying temperature can cause what is commonly called "the popcorn effect"--the fissuring and cracking and disintegration of the coal particles. Drying the coal and removing the moisture at a slower rate can reduce this effect--preventing disintegration and allowing moisture to escape in a manner that reduces cracking and fissuring. Smaller fissures make the coal particles more amenable to surface treatment agents which block the pores or coat the particle uniformly and act as effective oxidation or moisture readsorption inhibitors. However, slower drying rates necessitate longer residence times in the drying chamber to achieve the same degree of moisture removal. Because of the turbulent action in a fluidized drying bed, longer residence time leads to increased mechanical size-degradation of the coal particles, increasing dust in the dried product.